Marvin is mentioned only twice, during Petscop 5 & 8. In Petscop 5, after Paul asked why he should turn off the playstation, the pink Tool responds “MARVIN PICKS UP TOOL HURTS ME WHEN PLAYSTATION ON”. Marvin is also mentioned in Petscop 8, when Paul follows the green monster man and calls him "Marvin." As there is not much known about Marvin, the following theories may be shaky.
According to the message, Marvin is hurting whoever is speaking through pink Tool. From this we can conclude he is an evil man. If you believe in the Color Theory, you could say it’s Mike that’s being hurt by Marvin. The only other “character” we have been introduced to that seems to have evil tendencies is the man mentioned in the note in Care’s room. If this is true, then Marvin might also be the one responsible for Care’s downfall as well as other children.
The phrase “MARVIN PICKS UP TOOL” is still under discussion. It could be that Marvin is using Tool to hurt the child, or the “picking up tool” could be unrelated to how the child is being hurt. This message might also be referring to the smaller Tool-like objects found in Care and Mike’s bedrooms. As we are still unsure of the purpose of these objects, we can’t say for sure if they can be used to hurt someone.
On an even darker note, "Tool" is also sometimes used as a euphemism for male genitalia.
The fact that Marvin only hurts the child when the Playstation is on could indicate that the “people” in the game might not exist as long as the Playstation is off.